Electrodes for electron discharge tubes



Dec. 24, 1957 A. WEBER ELECTRODES FOR ELECTRON DISCHARGE TUBES Original Filed April 6, 1954 F/EQ .3

INVENTOR. ANTOD' LJEBER BY mww United States Patent ELECTRODES FOR ELECTRON DISCHARGE TUBES Anton Weber, Berlin-Steglitz, Germany, assignor to Telefnnken Gesellschaft fur Drahtlase Telegraphic, m. b. H., Hannover, Germany Original application April 6, 1954, Serial No. 421,223. Divided and this application November 17, 1954, Serial No. 478,463

Claims priority, application Germany April 10, 1953 7 Claims. (Cl. 316--6) The present invention relates to improved electrodes for electrical discharge vessels and more particularly to anodes for'electron discharge tubes.

it is known that electrodes which do not serve for electron emission and which during operation are subjected to a high thermal load may have imparted thereto a high heat radiation capacity to allow for loss of the heat by providing a surface thereof or at least the face thereof which faces the cathode with properties approaching the heat radiation properties of a black body.

These radiation properties may be achieved by producing the electrodes of iron or nickel sheet material, plating the same with aluminum, and after installation of the electrode into the vacuum vessel, for example, the electron tube, heating to about 800 C. There is formed by an exothermic reaction between the aluminum and the underlying metal an intermetallic union having such finely granular structure that its surface has about 80% or more of the radiation capacity of black bodies. If this reaction, the so called aluminizing reaction, is carried out in the vacuum, there is achieved in addition to the aluminizing a good degassing of the surface of the material, which egassing is necessary for proper operation of the mate rial as an electrode.

Although the above should theoretically result in the production of satisfactory electrodes, several disadvantages have been found which prevent the commercial use thereof and the production of satisfactory electrodes for vacuum vessels.

The base metal sheet, e. g. the iron or nickel sheet, is plated with the original aluminum layer either by a hot rolling or a cold rolling process. If a cold rolling process is utilized, the heat caused by the deformation of the metal is utilized to cause adherence of the aluminum layer to the underlying metal sheet. In any event, the cold rolling or the hot rolling to cause adherence of the aluminum layer to the underlying metal resultsin the formation of a relatively hard composite sheet which requires a raised temperature, i. e. soft annealing, in order to be able to deform the sheet into the shape of the desired electrode.

However, the soft annealing which is utilized to shape the composite sheet of base metal such as iron or nickel and aluminum layer into the shape of the electrode has the effect of aluminizing the underlying metal, e. g. forming an intermetallic union or alloy between the underlying metal and the aluminum layer. This is undesirable because the aluminizing should only take place in the vacuum vessel, e. g. the electron tube, in order to obtain the above-mentioned degassing of the electrode. Without such degassing the electrode does not operate efficiently. lt is apparent that the shaping of the composite sheet into the shape of the electrode cannot be accomplished in the vacuum tube itself in order to simultaneously achieve the degassing, because of the difficulties of operation for shap ing in the confines of the vacuum tube. Itis also assumed to be known as a general fact that premature aluminizing during soft annealing can be avoided by the use of a core metal particularly rich in oxygen, but this in turn is undesirable from the viewpoint of evacuating technique.

It is therefore a primary object of the present invention to provide a new material adapted to be made into electrodes for electron discharge tubes which avoids the abovementioned disadvantages.

it is a further object of the present invention to provide a material which may be shaped into the form of an electrode for an electron discharge tube and aluminized and degassed in the vacuum tube.

it is a still further object of the present invention to provide a process of producing such material which may be made into electrodes for electron discharge tubes.

It is another object of the present invention to provide a process of producing degassified electrodes for electron discharge tubes having an aluminized surface with the radiation capacities of a black body.

Other objects and advantages of the present invention will be apparent from a further reading of the specification and of the appended claims.

With the above objects in view, the present invention mainly comprises a new article of manufacture adapted to be made into electrodes for electron discharge tubes comprising, in combination, a sheet of a metal adapted to be aluminized upon heating thereof in contact with aluminum, the sheet of metal having an aluminized surface zone, and an aluminum layer superimposed upon and adhering to the aluminized surface zone of the sheet of metal.

The process of the present invention mainly comprises a process of producing electrodes for electron discharge tubes, comprising the steps of forming a first layer of aluminum on the surface of a sheet of a metal adapted to be aluminized, heating the metal sheet with the aluminum layer thereon at a temperature sutficiently high to alumi nize the metal and thereby form an aluminized surface zone in the metal sheet, superimposing on and uniting to the aluminized surface zone of the metal sheet a second layer of aluminum at a temperature below the aluminizing temperature of the metal and deforming the thus formed composite metal sheet at a temperature below the aluminizing temperature of the metal to the shape of an electrode for an electron discharge tube, and heating the electrode under vacuum at a temperature sufliciently high to aluminize the metal with the second aluminum layer and to degasify the electrode, thereby forming a degasified electrode.

The first aluminum layer is formed on the base metal sheet, e. g. of iron or nickel, by a hot or a cold rolling process which welds the aluminum to the underlying metal. Heat is then applied at a sufiicient temperature and for a sufiicient length of time to completely aluminize the surface layer of the sheets. A second layer of aluminum is then superimposed on and united to the aluminized surface zone of the metal sheet, e. g. by cold rolling, at a temperature below the aluminizing temperature of the metal sheet.

It is to be understood that the terms aluminize, aluminized and aluminizing are meant to connote the formation of an intermet'allic union between the aluminum layer and the underlying metal such as iron or nickel to form an aluminum composition, for example in the case of iron, FeAl The term aluminizing temperature" is used to designate the temperature at which such inter- Inetallic union is formed, this temperature varying somewhat depending upon the underlying metal with which the aluminum forms the intermetallic composition.

It is preferred that the composite metal sheet consisting of the sheet of metal having an aluminized surface zone and an aluminum layer superimposed upon and ad hering to the aluminized surface zone of the sheet of metal be critically deformed. The critical deformation of the composite sheet has the advantage of producing a rapid and considerable growth of the crystals of core metal during the aluminizing step. The term critical deformation is meant to denote the approximate degree of deformation at which the metal tends to produce crystals of a considerable size under the influence of a following annealing process. The critical deformation of the composite sheet results in transferring the same into a condition in which later heating to red hot temperature of the electrodes produces a rapid and con siderable growth of the crystals. When the heat treatment is carried out in vacuum for the purpose of degassing the electrodes, the enclosed gas is very rapidly expelled and a better degassing of the core portion as well is obtained in a fraction of the degassing time required for materials which were not critically deformed. Prior to the present invention, such critical deformation was never used for composite materials, e. g. materials wherein one metal is plated with a second metal.

The use of such critical deformation with the composite materials of the present invention is only possible because of the provision of an aluminized zone between the basic metal, i. e. iron or nickel, and the aluminum layer. If a composite sheet of a metal such as iron and aluminum is utilized, without the aluminized zone between, it is necessary to apply a very high roller pressure in order to obtain an intimate connection of the aluminum layer with the base metal. This high roller pressure in the case of iron or nickel and aluminum is at least 20%, which is far above the critical deformation of only about 8%. Also, thorough deformation must be applied if the basic material to be coated with aluminum is an iron sheet which has a carbon content of 0.01-0.5% and contains 0.01-1% of metallic aluminum as killing additive to the melt, and which is therefore practically free of oxygen. It is advantageous to use such iron from the vacuum technique point of view.

As mentioned previously, in composite sheets having a single aluminum coating without the aluminized zone between the aluminum coating and the underlying base metal, it is necessary to carry out the softening annealing for shaping the composite sheet in such manner that during the annealing no aluminizing takes place. This I condition limits the permissible temperature range for the heating to soft annealing and moreover requires a relatively long time of heating since only a comparative- 1y low temperature of less than 600 C. is permissible so that there is a suflicient temperature range between the annealing temperature and the aluminizing temperature, i. e. the temperature at which formation of the intermetallic compound FeAl or NiAl takes place spontaneously.

This disadvantage is overcome by the present invention. The soft annealing of the core metal with the first aluminum layer to the underlying metal is carried out at the usual temperature of about 675 C. at which temperature the aluminum coating is completely transformed into an intermetallic union. In other words, this temperature may be considered as the aluminizing temperature, and the first aluminum layer may be joined to the underlying metal by formation of the aluminized surface zone. It is thus unnecessary to maintain a predetermined temperature during the heating to soft condition and consequently this may be accomplished rather quickly.

It is onto this completely aluminized surface zone that according to the present invention a second aluminum layer is united, i. e. by cold rolling. It has been found according to the present invention that this second aluminum layer firmly adheres to the underlying aluminized surface zone utilizing a considerably lower rolling pressure that would be necessary to cause an aluminum layer such as the first aluminum layer to adhere to the basic metal per se without the intermediate. aluminized zone. It is therefore possible to easily and quickly accomplish this uniting of the second aluminum layer to the aluminized surface zone of the metal sheet at temperatures below the aluminizing temperature.

Moreover, it is possible by virtue of the intermediate aluminized zone to utilize sufficiently low pressures during the rollingso as to be able to adjust the rolling pressure in such manner that critical deformation of the base metal takes place, thereby assuring the most rapid possible growth of the crystals during the later heating for aluminizing and the degassing. Thus, the critical deformation of approximately 8% in iron, and even less in nickel is suflicient to result in adherence of the aluminum coating to the underlying metal by means of the intermediate aluminized zone.

Due to the much lower deformation of the base material during the second coating, namely upon coating the second aluminum layer to the aluminized surface zone of the metal sheet, the further advantage is obtained that the formed composite metal sheet is not as hard as that obtained upon formation of the first aluminum layer directly to the underlying metal utilizing deformation which is of necessity higher than the critical deformation pressure.

It is therefore possible to easily shape the composite metal sheets of the present invention consisting of a base metal having an aluminized surface zone and an aluminum layer adhering to the aluminized surface zone into electrodes whereby the previously mentioned advantage of rapid crystal growth during the heating and degassing of the electrodes in the vacuum tube is attained.

However, even aside from this advantage, and even if a softer material is desired, the double coating according to the present invention is of advantage because the final softening by heating can be more easily carried out without the danger of premature aluminizing of the second aluminum coating. This is probably due to the fact that the intermetallic layer, the aluminized surface zone of the base metal, slows down the reaction which leads to aluminizing between the underlying base metal and the second aluminum coating, and consequently during the heating to soft condition the operation can be carried out within a wider temperature range and at a higher temperature than is possible with materials having only a single aluminum layer without the aluminized zone between.

Moreover, the surface of such composite material may be transformed completely into an intermetallic compound, namely an aluminized layer, during the degassing heating whereby the resulting material has at least as good a radiation capacity as that obtained with single plated composite sheets. Since the transformation of the second aluminum coating into the intermetallic composition does not take place in an explosion-like manner, but takes place slowly, it may be more easily controlled by suitable control of the heating during the degassing heating of the present invention. During the degassing by means of a vacuum pump, it is desirable that the increase of gas pressure accompanying the aluminizing takes place as slowly as possible. The present invention therefore provides an additional embodiment, which allows for bet ter control of the degassing in the vacuum tube.

According to this embodiment, which will be more fully described later, the composite metal sheet consisting of the base metal having an aluminized surface zone and the second aluminum coating adhering to the aluminum surface zone is, before introduction into the vacuum tube, heated for a while at a temperature below the aluminizing temperature, e. g. at a temperature of about 500 C. preferably at atmospheric pressure or below in an atmosphere of air, hydrogen, nitrogen, lighting gas, ammonia, or the like, to cause a partial degassing so that the later final degassing during the aluminizing in the vacuum tube may be more easily controlled.

This preliminary heat treatment below the aluminizing temperature does not result in any soft annealing of the material but only in a driving-01f of some of the gas so thatthe degassing of the electrode or other constituent of the electrode construction produces a lesser amount of free gas and thereby results in a lesser increase of the gas pressure.

In order to better understand the preliminary heating step of the composite metal sheet of the present invention before introduction thereof into the vacuum tube for final aluminizing and degassing, it should be noted that during the rolling on of the second aluminum. layer onto the aluminized and therefore porous underlayer, air is enclosed. This enclosed air must be. driven off in the vacuum vessel during the degassing heating. This results in a relatively violent gas eruption which greatly increases the load on the vacuum pump and increases the pumping time.

By the above described embodiment of the present in vention of providing a preheating below the aluminizing temperature before introduction into the vacuum tube,

a large portion of the enveloped air is driven off so that during the aluminizing and degassing heating of the electrode in the tube only a small amount of the gas need be driven off. Tests have shown that moreover the remaining gas content by the application of the preheating of the material before introduction into the vacuum tube is lower and therefore the removal of the last traces of gas, e. g. by getter is facilitated.

The preliminary heating of the composite aluminum coated metal sheet having the aluminized zone between the base metal and the aluminum coating, must be carried out at below the aluminizing temperature of the base metal to the aluminum layer, and is preferably carried out at about 500 C. Since the aluminizing of the surface layer is retarded by the already aluminized intermediate zone, the choice of the heating temperature and the duration of the heating treatment, which for example may be /2 hour, has a wide latitude.

The value of this heat treatment may be illustrated by the following tests: If a sample weighing about grams and being plated on both sides is placed in a closed container having a 500 cc. capacity and being provided with a high vacuum pump and heated to a temperature at which, as in the vacuum tube, the underlying metal is aluminized with the aluminum surface coating, i. e. the basic metal forms an intermetallic compound zone with the aluminum coating, there escapes an amount of 1.2 cc. of gas and after a predetermined time a remaining gas pressure of 0.0023 mm. which must be compensated for in a vacuum tube by getter. If the same test is repeated with the same sample but which according to the preferred embodiment of the present invention is first heated for a half hour at a temperature of 500 C., only 0.26 cc. of free gas is obtained and the remaining gas pressure after the same time of treatment as before is only 0.0013 mm.

The double plating, namely the plating of a layer of aluminum to a sheet of metal which already has had a layer of aluminum plated thereto and then aluminized may be carried out on only one side or on both sides of a metal sheet. In the event that only one side of the sheet is so aluminum plated, it is possible to leave the other side of the sheet glossy and coat the same with another material, for example nickel. The basic material which is to be coated twice by aluminum may itself be a composite sheet, for example, nickel with an iron coating. Any suitable metal may be used for the base metal sheet which comes in contact with the first aluminum layer, it being understood of course, that this base metal must be capable of being aluminized upon heating in contact with alumi mum, in other words that it be capable of forming an intermetallic composition upon heating with aluminum.

The above mentioned embodiment of heating the composite sheet .to below the aluminizing, temperature before introduction of the same as an electrode in the vacuum tube is particularly advantageous for sheets which are plated on both sides with the double aluminum coating because the last remaining gas leaving the base. material appeared to be retained in the thermic activated aluminized intermediate layer and by this embodiment a reducing of this amount of gas takes place.

The novel features which are considered as characteristic for the invention are set forth in particular in the appended claims. The invention itself, however, both as to its construction and its method of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings, in which:

Fig. 1 is a cross section of a material according to the present invention having an aluminum coating adhering to an aluminized zone of a metal sheet on one side of the metal sheet;

Pig. 2 is a cross section having aluminum coatings on both sides of a metal sheet;

Fig. 3 shows the electrode material of the present invention inserted as an electrode in a vacuum tube and before degassing and aluminizing thereof; and

Fig. 4 shows the electrode of Fig. 3 after degassing and aluminizing.

Referring more particularly to the drawings it may be seen that a base metal 1 of iron or nickel has an aluminized zone 2 consisting of an intermetallic composition such as FeAl or NiAl, and a layer of aluminum 3 adhering to the aluminized zone 2. In Fig. 2 the sheet of metal 1 has aluminized zones 2 on each side and layers of aluminum 3 adhering to each of the aluminized zones.

In Fig. 3 a vacuum tube 4 having a cathode 6 is provided with an anode (which is greatly enlarged in the drawing for better understanding of the invention) consisting of the base metal 1 and aluminized zone 2 and an aluminum coating 3. In Fig. 4 the anode shown in the vacuum tube of Fig. 3 has been subjected to an aluminizing and degassing heat treatment so that the aluminum coating 3 in Fig. 3 forms with the aluminized zone 2 a single aluminized surface zone 5.

The following example is given to illustrate the process of the present invention, the scope of the invention not however being limited to the specific details of the example:

Example A sheet of metal having a thickness of about 3 mm., the metal preferably consisting of an oxygen-poor iron preferably having a carbon content of about 0.01-0.5% and having metallic aluminum in an amount of 0.01- 1% added to the molten material, is by a rolling welding process coated on each side with an aluminum foil having a thickness of 0.1 mm. The composite sheet is then rolled down to a composite thickness of about 0.15 mm. whereby the aluminum layer also becomes correspondingly thinner.

Thereupon the thus obtained composite sheet is heated to soft condition at a temperature of about 675 C. so that the aluminum coating is completely transformed into an intermetallic composition with the base metal. The thickness of the aluminum layer after the rolling step may be chosen to be smaller or greater, and is preferably about 3-10 microns. Depending upon. the thickness of the aluminized layer, the aluminizing of the second aluminum layer applied is to a greater or lesser extent slowed down.

To the aluminized zone there is applied a second aluminum layer preferably having a thickness of about 15 microns. This second aluminum layer is applied to the composite sheet by rolling preferably to that extent corresponding to the critical deformation of. the underlying metal. The degrees of critical deformation for :pure iron is approximately 8%, but the rangein which critical deformation takes place is wider and its limits are not exactly determined by values which are valid for all metals and their alloys.

While it is generally necessary to dull the sheet surface before the rolling on of a single aluminum layer if practically oxygen-free iron is used, such dulling step may be omitted in the application of the second alumi num layer to the aluminize-d surface. This fact might be explained by the theory that the aluminizing step produces an intimate connection of the first coating with the base material and on the other hand produces a rough the electrodes are formed by stamping and bending and then placed into the electrode system. During the heating to red hot temperature for degassing which is carried out on the finished electrode system already built into the vacuum vessel after closing the envelope and evacuating the tube, the aluminizing of the second aluminum layer takes place accompanied by a rapid growth of the crystals.

'It will be understood that each of the elements described above, or two or more together, may also find a useful application in other types of electrode constructions differing from the types described above.

While the invention has been illustrated and described as embodied in electrodes for electron discharge vessels, it is not intended to be limited to the details shown, since various modifications and structural changes may be made without departing in any way from the spirit of the present invention.

Without further analysis, the foregoing will so fully reveal the gist of the present invention that others can by applying current knowledge readily adapt it for various applications without omitting features that, from the standpoint of prior art, fairly constitute essential characteristics of the generic or specific aspects of this invention and, therefore, such adaptations should and are intended to be comprehended within the meaning and range of equivalence of the following claims.

What is claimed as new and desired to be secured by Letters Patent is:

1. In a process of producing electrodes for electron discharge tubes, the steps of forming a first layer of aluminum on the surface of a sheet of a metal adapted 'to be aluminized; heating said metal sheet with the aluminum layer thereon at a temperature sufficiently high to aluminize said metal and thereby form an aluminized surface zone in said metal sheet; and superimposing on and uniting to said aluminized surface zone of said metal sheet a second layer of aluminum at a temperature below the aluminizing temperature of said metal, thereby forming on said aluminized surface zone of said metal sheet a non-aluminized coating consisting essentially of aluminum adhering to said aluminized surface zone.

2. 'In a process of producting electrodes for electron discharge tubes, the steps of forming a first layer of aluminum of the surface of a sheet of a metal adapted to be aluminized; heating said metal sheet with the aluminum layer thereon at a temperature sufficiently high to aluminize said metal and thereby form an aluminized surface zone in said metal sheet; superimposing on and uniting to said aluminized surface zone of said metal sheet a second. layer of aluminum at a temperature below the aluminizing temperature of said metal, thereby forming on said aluminized surface zone of said metal sheet a. non-aluminized coating consisting essentially of aluminum adhering to said aluminized surface zone; and deforming the thus formed composite metal sheet at a temperature below the aluminizing temperature of said metal to the shape of an electrode for an electron discharge tube.

3. A process of producing electrodes for electron discharge tubes, comprising the steps of forming a first layer of aluminum on the surface of a sheet of at least one lit 8 metal selected from the group consisting of iron and nickel; heating said metal sheet with the aluminum layer thereon at a temperature sufiiciently high to aluminize said metal and thereby form an aluminized surface zone in said metal sheet; superimposing on and uniting to said aluminized surface zone of said metal sheet a second layer of aluminum at a temperature below the aluminizing temperature of said metal, thereby forming on said aluminized surface zone of said metal sheet a nonaluminized coating consisting essentially of aluminum adhering to said aluminized surface zone and deforming the thus formed composite metal sheet at a temperature below the aluminizing temperature of said metal to the shape of an electrode for an electron discharge tube; and heating said electrode under vacuum at a temperature sufficiently high to aluminize said metal with said second aluminum layer and to 'degasify said electrode, thereby forming a degasified electrode.

4. A process of producing a degasified electrode in an electron discharge tube, comprising the steps of forming a first layer of aluminum on the surface of a sheet of a metal adapted to be aliuninized; heating said metal sheet with the aluminum layer thereon at a temperature sufficiently high to aluminize said metal and thereby form an aluminized surface zone in said metal sheet; superimposing on and uniting to said aluminized surface zone of said metal sheet a second layer of aluminum at a temperature below the aluminizing temperature of said metal, thereby forming on said aluminized surface zone of said metal sheet a non-aluminized coating consisting essentially of aluminum adhering to said aluminized surface zone and deforming the thus formed composite metal sheet at a temperature below the aluminizing temperature of said metal to the shape of an electrode for an electron discharge tube; inserting the thus formed electrode into an electron discharge tube; closing the envelope of said electron discharge tube evacuating said electron discharge tube; and heating said electrode under Vacuum in said electron discharge tube at a temperature sufficiently high to aluminize said metal with said second aluminum layer and to degasify said electrode, thereby forming a degasified electrode in said electron discharge tube.

5. A process of producing electrodes for electron discharge tubes, comprising the steps of forming a first layer of aluminum on the surface of a sheet of a metal adapted to be aluminized; heating said metal sheet with the aluminum layer thereon at a temperature sufficiently high to aluminize said metal and thereby form an aluminized surface zone in said metal sheet; superimposing on and uniting to said aluminized surface zone of said metal sheet a second layer of aluminum at a temperature below the aluminizing temperature of said metal, thereby forming on said aluminized surface zone of said metal sheet a non-aluminized coating consisting essentially of aluminum adhering to said aluminized surface zone and deforming the thus formed composite metal sheet at a temperature below the aluminizing temperature of said metal to the degree of critical deformation of said metal and forming said composite metal sheet to the shape of an electrode for an electron discharge tube; and heating said electrode under vacuum at a temperature sufiiciently high to aluminize said metal with said second aluminum layer and to degasify said electrode, thereby forming a degasified electrode.

6. A process of producing electrodes for electron discharge tubes, comprising the steps of forming a first layer of aluminum on the surface of a sheet of a metal adapted to be aluminized; heating said metal sheet with the aluminum layer thereon at a temperature sufliciently high to aluminize said metal and thereby form an aluminized surface zone in said metal sheet; superimposing on and uniting to said aluminized surface zone of said metal sheet a second layer of aluminum at a temperature below the aluminizing temperature of said metal,

thereby forming on said aluminized surface zone of said metal sheet a non-aluminized coating consisting essentially of aluminum adhering to said aluminized surface zone; heating the thus formed composite metal sheet at a temperature below the aluminizing temperature of said metal so as to partially degas said composite metal sheet; and further heating said partially degassed metal sheet under vacuum at a temperature sufiiciently high to aluminize said metal with said second aluminum layer and to degasify said electrode, thereby forming a degasified electrode.

7. A process of producing a degasified electrode in an electron discharge tube, comprising the steps of forming a first layer of aluminum on the surface of a sheet of a metal adapted to be aluminized; heating said metal sheet with the aluminum layer thereon at a temperature sufiiciently high to aluminize said metal and thereby form an aluminized surface zone in said metal sheet; superimposing on and uniting to said aluminized surface zone of said metal sheet a second layer of aluminum at a temperature below the aluminizing temperature of said metal, thereby forming on said aluminized surface zone of said metal sheet a non-aluminized coating consisting essentially of aluminum adhering to said aluminized surface zone and deforming the thus formed composite metal sheet at a temperature below the aluminizing temperature of said metal to the shape of an electrode for an electron discharge tube; heating said electrode at a temperature below the aluminizing temperature of said metal so as to partially degas said electrode; inserting the thus formed electrode into an electron discharge tube; closing the envelope of said electron discharge tube; evacuating said electron discharge tube; and heating said electrode under vacuum in said electron discharge tube at a temperature sufficiently high to aluminize said metal with said second aluminum layer and to degasify said electrode, thereby forming a degasified electrode in said electron discharge tube.

References Cited in the file of this patent UNITED STATES PATENTS 

